Typically, low-Earth-orbit (LEO) satellites orbit at a height from the Earth that varies approximatively between 400 and 800 km, are generally equipped with Earth observation systems, such as synthetic aperture radars (SARs) and/or optical instruments, and are configured to transmit remotely-sensed data to ground stations by means of microwave antennas. The transmission from LEO satellites to ground stations of data remotely sensed by on-board Earth observation systems is generally referred to as data downlink (DDL) and antennas used for this function are generally known as DDL antennas.
Moreover, special ground stations, typically called Telemetry, Tracking and Control (TT&C) stations, are used to monitor and control operation of LEO satellites. In general terms, TT&C stations receive telemetry data from LEO satellites to monitor operation thereof, and transmit commands to LEO satellites to control operation thereof and ranging signals to track said satellites. Therefore, LEO satellites need to be equipped also with TT&C antennas for TT&C data exchange.
As is known, current LEO satellites are equipped with two separate antennas for DDL and TT&C, respectively. This fact causes installation problems, especially on board LEO satellites fitted with large antennas and/or appendages (such as solar arrays, booms, supports, instruments, etc.), since both DDL and TT&C antennas require a very large field of view.
Nowadays, all European LEO satellites for Earth observation use S and X bands almost exclusively for TT&C and DDL (as broadly known, the S band being defined as the microwave portion of the electromagnetic spectrum including frequencies ranging from 2 to 4 GHz, while the X band being defined as the microwave portion of the electromagnetic spectrum including frequencies ranging approximatively from 7 to 12 GHz), but these bands are becoming more and more congested due to their the massive use. For this reason, a portion of K band (as broadly known, the K band being defined as the microwave portion of the electromagnetic spectrum including frequencies ranging from 18 to 27 GHz) has been recently allocated for DDL in order to increase downlink throughput capability of LEO satellites, wherein said new K-band portion allocated for DDL includes frequencies ranging from 25.5 to 27 GHz.
Additionally, a new X-band frequency allocation has been proposed for TT&C by the International Telecommunication Union (ITU) at the World Radiocommunication Conference 2015 (WRC-15) in relation to the Earth Exploration Satellite Service (EESS), including the frequency range 7190-7250 MHz for the TT&C uplink. This new uplink allocation can be used in combination with the existing EESS allocation of the frequency range 8025-8400 MHz for the TT&C downlink.
As is known, current TT&C antennas operating in S or X band are usually based on helix-type antennas or biconical antennas, while current solutions for fixed DDL in X band from LEO satellites mainly employ helices or parasitic coaxial horns. In this connection, it is worth noting that wire-type antennas (i.e., helices or wire-based solutions) are not applicable to the new K-band portion allocated for DDL due to technological problems and limited power handling capability (in particular, due to thermal problems and corona discharge). Moreover, parasitic-coaxial-horn-type solutions for DDL are currently limited by a low level of cross-polarization discrimination, well above the acceptable level for dual-polarization frequency reuse (i.e., higher than 20 dB cross-polarization discrimination).